Compass correction system



1111- 1952 T. o'c. M CARTHY COMPASS CORRECTION SYSTEM 2 SHEETS-SHEET 2Filed July 11, 1949 FIG. 4

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Patented Jan. 8, 1952 UNITED STATES PATENT OFFICE May 13, 1944. SerialNo. 104,122

24 Claims.

(Granted under the act of amended April 30, 1928;

This invention relates to electrical systems for transmitting angularmotion that include a transmitter unit and one or more repeater units.

This application is a continuation of my copending application entitledCompass Correction System, Serial Number 535,529, filed May 13, 1944,abandoned.

A well-known type of such a system includes transmitter and repeaterunits which are usually alike and each comprises a single circuit fieldwinding and a polycircuit armature winding, one of which constitutes astationary element known as the stator and the other a movable elementknown as the rotor. The field windings of the transmitter and repeaterare excited from a suitable source of alternating current and therespective armature windings are connected toether.

These systems are used for many purposes and, under normal conditions,the repeater unit will follow the transmitter unit faithfully. In otherwords, motion of the transmitter through a 30 angle will cause therepeater to move through a 30 angle in the same direction, and so on.

In certain applications of angular-motiontransmission systems, it isdesired not to have the repeater follow the transmitter degree for Thisa pplication July 11, 1949,

degree. For example, in a remote-reading compass system it is desirableto introduce a correction for the well-known compass errors. Likewise indirection-transmitting systems such as used in ordnance for targetdesignation, the transmitted direction must be modified to allow forhorizontal and for vertical parallax before the gun, range finder, orSearchlight will point at the target being designated by the locatorinstrument. Similarly, in radio direction finders there exists an errorthat resembles a deviation error found in the compass art and that is,in fact, called a deviation error. This error can be corrected,compensated for, or removed so as to provide a direction indication thatis approximately true. Many other installations incorporatingdirection-transmission systems are known wherein it is likewisedesirable to provide a controllable asynchronous relation between thetransmitter and repeater units.

The general object of this invention therefore is to provide a new andimproved apparatus for efiecting a controllable asynchronous relationbetween the transmitter and the one or more repeater units of anangular-motion-transmitting system.

Another object is to provide an apparatus of the type described whereinthe amount of the asynchronism between the transmitter and repeaterunits varies as a function of the instant angular position of thetransmitter unit.

March 3, 1883, as 370 0. G. 757) A further object is to provide a newand im- 7 proved electrical device for connection between thetransmitter and repeater units of such a system wherein the normalsynchronous relation existing between these units is made asynchronousby an amount that varies as a desired function of the instant angularposition of the transmitter unit relative an external field.

Another object of this invention is to provide an improved and automaticelectrical compensator for angular-motion-transmission systems that ismuch less costly than the present known mechanical compensators and thatis also exceedingly simple in construction and operation. Furthermore,through simple adjustment means, the character of the compensationeffected by the device can be altered over a very wide range to providealmost any action required.

Another object is to provide a new and improved apparatus thatautomatically compensates for the deviation error in remote-readingcompass systems.

A further object of the invention is to provide a new and improvedapparatus that automatically compensates for the deviation error in anearth-inductor-compass system.

A still further object of the invention is to provide a novel type ofconstruction for a control transformer.

Another object is to provide a novel means of combining electrical andmechanical quantities.

These and other objects of the invention will become more apparent fromthe detailed description to follow and the accompanying drawings whichshow preferred embodiments of the invention in which Fig. 1 is adiagrammatic view of the invention as applied to a flux-gateearth-inductor-compass system;

Fig. 2 is a view of the primary and secondary coil arrangement in oneleg of the flux-gate;

Fig. 3 is a perspective view of the gyro-stabilized flux-gate;

Fig. 4 is a diagrammatic view of a modified form of the electricalcompensator shown in Fig.

Figs. 5, 6 and 7 are plots of the deviation, correction and resultantcurves for the apparatus in Fig. 1; and

Fig. 8 is a view showing a still further modified form of compensator.

In Figs. 1-3, inclusive, there is shown diagrammatically anearth-inductor-compass system according to this invention. A saturablecore transformer, or flux-gate, indicated by numeral Ill, consists ofthree legs, ll, i2 and [3, arranged to form an electrical equilateraltriangle in space.

All of these legs are of the same construction and each of them (seeFig. 2) such as leg I consists of a primary winding I Ia, secondarywinding I Ib, and a pair of" closely adjacent. cores I I and Iid.

As shown in Fig. 2, the primary winding Hat is divided, one half beingwound on core He and the other half on core lid. The two. halves of theprimary are wound in opposite directions and they are thusnon-inductive. The primary wind ing IIa is excited from a source ofalternating current which, in the present embodiment, has a frequency of487 cycles obtained from a suitable oscillator I l of conventionaldesign and therefore shown only in block diagram.

The primary windings I la, I2a, and [3a aredesigned to saturate theirrespective cores twice during each cycle, and for most of the cycle.During the saturation period there is, of course, notransformer actionbetween the primaries and their associated secondaries. During each ofthe two unsaturated periods of the cycle, there is still no transformeraction between the primary and secondary windings insofar as concernsthe component of current in the primary from the. 487 cycle sourcebecause the two halves of each primary winding are Wound in oppositedirections. However, during each of the unsaturated periods, the earthsflux cuts through both halves of the core in the same direction andtherefore induces a voltage in each secondary winding have a frequencyof twice that of the primary, or 9'75 cycles.

As shown in Fig. 3, the three legs of flux-gate. It may be enclosed in acasing which is mounted in gimbals and stabilized by a gyro in order tomaintain it horizontal in the earths field.

The amplitude of each ofthe three voltages induced inthe secondaries Ilb, I21) and I312 will of course depend on their instant positionrelative to the direction of the earths field. These voltages willtherefore vary with the instant heading of the aircraft, vessel or othercarrier upon which the flux-gate is mounted. As the compass is rotatedin the earths flux, the three voltages will vary in a manner similar tothe voltage variations caused bymovementof the rotor in a conventionalsynchro system.

It should be pointed out that, the magnitude of the induced voltages in.these secondaries, which are delta connected, is very small, being ofthe order of a, few microvolts. Hence, it is necessary to connect theoutput thereof to a, Y- connected stator I5 of a coupling Autosyn I6.The rotor element I? of this Autosyn is provided with a single-circuitwinding and the voltage induced therein by the currents in itspoly-circuit stator windings is amplified. in amplifier I8, ofconventional construction, andtransmittedto the variable-phase windingI9 ofjthe two-phase stator element of a low-inertia, induction motor.The. other phase winding Ilia of the stator is connected to the 975cycle A. C. output terminals, on oscillator I4. Induction motor 20 has asquirrel-cage rotor 2 I.

Rotors I7 and 2| are mechanically coupled together by a shaft 22. Hencethe voltages impressed upon the stator windings I9 and I9a of motor 2Bwill cause its rotor 21 and, rotor ii to, rotate until the latterreaches its null position. Induction motor 20 is therefore a torqueamplifier which turns rotor II to the position it would take were itable itself to, develop the necessary torque.

From what has so far been described, itisseen that achange in angularposition .of, the flux;-

gate I0 relative to the earths field effects a like angular change inthe position of rotors I1 and 2I and the shaft 22. connectedtherebetween. The

flux-gate I0 is therefore analogous to the trans- ;r mitter unit of asynchro or self-synchronous system and the motors I6 and 20 analogous toa repeater unit. of such system.

For repeating the instant angular position of rotors I! and 2|, a,second self-synchronous system is utilized and comprises a transmitterunit 23. and one, or more repeater units 24.

The transmitter and repeater units are similar v in structure, althoughthe repeater would normally be smaller than the transmitter if thelatter is, to drive a number of repeater units.

The transmitter unit 23 includes a permanentmagnet rotor 25 coupled toshaft 22 and a stator including a circular, laminated core. The statorhas an exciting coil 21; wound upon it with a lead tapped off at each120- point, thus making four leads altogether: two, input leads and" twotapped leads. The constants of the transmitting unit (the number ofturns, value of exciting current, and magnetic alloy selected for thestator, core). are chosen so that the stator core will be completelysaturated twice during; and for most of, each cycle of its supply"source which has a free quency of 400 cycles. Duringperiods ofsaturation, no inductive effect can be produced within the core by theexciting current or the permament-magnet rotor. However, during-each ofthe short unsaturated periods of the core, the rotor is free to producean inductive effect upon it. During these periods, the rotors magneticflux flows through the core, and as it does so, induced voltages aresuperimposed in the stator winding; These voltages are at a frequency of800' cycles since the stator core is rendered unsaturated for a shortperiod twice during each cycle. The voltages across the taps of each ofthe three parts of the stator winding differ in value and vary with theinstant position of the rotor 25' relative to stator winding 21.

As previously stated, the repeater unit 2 3 is simwinding are identifiedby reference numerals 28 and 2 9, respectively;

The stator windings 2;! and 29 of the two unitsare connected in.parallel and excited fromthe same 400 cycle A. C. source.

When the rotors of both units arein the same position relative to theirrespective stator windings, the three tapped voltages. induced in thestator windings 21 and 29 will be alike, However, when the rotor 25 isrotated by shaftZZ, the

induced voltages in the. stator winding 2] will differ frorn' those.inlthe stator winding 29 causing signal current to flow from winding 2'?to winding 29 superimposed upon the excitation current. This flow ofcurrent produces a, new

resultant magnetic field flux in. the stator wind ing 29 causing therotor 28 of the repeater unit to rotate until itis in the same, relativeposition to its stator winding 29 as the rotor 25,, in the, transmitterunit is to its staor winding 21''. Accordingly, any angular displacementof rotor 25 by rotation. of shaft, 22' causes a like displacerment ofrotor 28.

It is now, seen thatv as. the bearing of the car- 0 rier for theflux-gate Ill changes relative to v the.

earths field, such bearing change will be fed electrically through the.Autosyn coupling I5 and induction motor 20 producing an equal angularchange in the position of shaft 22 and of the 76 rotor 25 in the.transmitter unit, 23, which anguilar to transmitter unit 23 and itsrotor and stator lar change may be indicated by a pointer 30.Transmitter unit 23 is then, in effect, a master compass. The positionof pointer 30 may then be repeated by a pointer 3| on the rotor elementof the one or more repeater units 24.

The earth-inductor-compass and all other types of compass systemsdepending upon the The remote-reading earth-inductor compass system thathas been described is exceedingly practical since the magneticallysensitive element (flux-gate may be mounted so as to eliminate theeffects of the unsymmetrical iron and steel of the compass carrier, suchas are found in a vessel, and in a location where the efiects ofhorizontal soft iron will be of a low value. When this condition isobtained, the deviation error is reduced to a factor, that when plottedas an ordinate against compass indication as an abcissa, results in acurve having sinusoidal characteristics. A typical such curve is shownin Fig. 5.

In the earth-inductor-compass that has been described, the bearingindication on the transmitter unit 23 would, if not compensated, be inerror by the algebraic sum of the variation and deviation errors. Inthis invention no compensation for variation is contemplated becausethis component of the total error is a constant for any latitude inwhich the compass carrier may be. However, one Way for compensating thevariation error would be to provide some means for shifting the rotor 25of the transmitter 23 relative to shaft 22.

However, this invention does provide an automatically operating andnovel corrector or compensator which will substantially cancel out thedeviation component of the total error.

Automatically operated deviation compensators presently known are of amechanical nature and most costly and in no case do they alter thecompensation to match the change in deviation error caused bydisplacement in magnetic latitude of the carrier. This inventionsubstitutes an electrically operated compensator that is far more simpleand less costly.

In particular, and with reference to Fig. 1, one form of my improveddeviation compensator comprises a Wave generator 32 and a controltransformer device 33. The wave generator 32 consists of a flux-gateelement which is similar to any of the three legs of device [0. That is,the generator 32 includes two cores 32a and 32b of a material having ahigh permeability with a pri-.

mary winding 32c split between them, the two halves of this windingbeing wound on their respective cores in opposite directions so as to benon-inductive with respect to the secondary winding 32d which surroundsprimary winding 320.

Primary winding 320 is fed from the same 487 cycle output terminals ofoscillator M as the primary of flux-gate l0. However, wave generator 32is designed to operate in the magnetic field of a permanent magnet 34,which is placed so as to provide a flux field of the proper in- Thedeviation is,

63 tensity and direction. The earths field is obviously also present inthe cores 32a and 3212 but its intensity, when compared to that ofmagnet 34, is so minute that it can be totally disregarded so far asoperation of generator 32 is concerned.

Like flux-gate it the two cores 32a and 32b are saturated twice during,and for almost the entire period of, each cycle. However, during the twoshort periods in each cycle when cores 320., 3212 are unsaturated, theentry of the magnetic field from magnet 34 into these cores effects atransformer action to induce a voltage into the secondary 32d. Thefrequency of the voltage output from the secondary 32d will be twicethat of the primary, or 975 cycles since the cores 32a, 32b aresaturated twice during each cycle.

Transformer 33 includes a rotatable primary winding 33a and a core 331)which may be circular. A distributed secondary Winding 330 is wound uponcore 332) and tapped at the points. The output from secondary winding32d feeds the primary 33a. Consequently, when current flows in primary33a, a voltage is induced in each of the three divisions of secondary330 between the 120 tap points, the amplitude of each such voltage beingdependent in part upon the particular angular position at Which therotatable primary winding 33a is set.

These three voltages, which are the correction voltages to compensatefor the deviation error, are then connected into the electricalconnections between the secondary windings Ill), 1211, i312 and statorwindings l5 of the coupling Autosyn I6 by means of conductors 35, 33 and31 and series-connected variable resistors 38, 33 and 49.

During the operation of the fluxgate system, a very complex flow ofcurrent exists between the secondaries of the fluxgate element It andthe stator elements H! of the coupling Autosyn l6. This current changesfor every angular position of the fluxgate with respect to the earthsfield. A part, at least, of the current flowing between the fiuxgateelement and stator element 15 is thought to circulate through thesecondaries of the transformer 33, reacting, in turn, upon the primaryof this transformer and causing the current flowing therein to vary instrength as the fluxgate element rotates in the earths field. The outputelectromotive force of the transformer unit, accordingly, varies as afunction of the angular displacement of the compass or earth inductor.

In the system shown in Fig. l, the primary winding 320 of generator 32is excited at one-half the frequency of the voltage outputs from.secondary windings I lb, I22) and I31) of the flux-gate I0. As thisfrequency is doubled by wave generator 32, the correction voltages fromtransformer 33 will be at the same frequency as the output voltages fromsecondaries 1 lb, mo and Nb. As the excitation for both flux-gate l0 andgenerator 32 is derived from the same source, oscillator H3, andtransformer 33 is electrically coupled to generator 32, apredeterminable but controllable phase relationship exists between thedeviation correction voltages and the output voltages from flux-gate ID.

It should now be apparent that if good compensation .for the deviationcomponent of the total compass error is to be obtained, correctionvoltages must be derived that, if applied to an earth-inductor-compasssystem without a compensator and located on a vessel or other carrierhaving no iron whatsoever, would produce a curve such as that shown inFig; 6. -=Theoretically, this curve. should be such that were it'- tobe. combined algebraically with the curve in Fig. 5, the net resultwould be zero error around theentire-hori'zon. Although thetheoretically desired: curve is pos sible, it has been found in practicethat. a.correc tion curve which only approximates the. contour of thedeviation curve and. will therefore produce the resultant curve such asthat shown in Fig; 7 is satisfactory. Such a curvecanbe obtained byvarying the position of the. primary 33a relativeto the secondary 33cassociated therewith and making. such adjustments as ma'y be necessaryin the settings of resistors 38, 3'9 and Gil in circuit with. thesecondary winding. of transformer 33, and. resistor ti in circuit withthe primary of generator 32.- The points atwhich the correctioncurvecrosses the base line can be adjusted by shifting the position ofprimary winding 33a and the amplitude of the curve isadju'sted throughresistors 3 ili l inclusive.

In other words, as the heading of the carrier on which the compasssystem is installed changes through 360, the only error will be thatshown in Fig. '7 which is within the inherent error of the compasssystem.

It should be noted that-once the correct positionfor the primary 33d oftransformer 33 relative to its secondary 330 is adjusted for anyparticular compass installation, it need not be disturbed because itwill automatically produce the right amounts of correction voltagesrequired at any instant bearing throughout the 360' range.

As an alternative arrangement for introducing the compensation necessaryto offset the deviation component of the total compass error", referenceis now made to Fig-4. Transformer 3-3" in Fig. 4 is simllar'totransformer33- shown in Fig. 1, containing. a rotatably adjust-ableprimary winding 33a, core 331) and a distributed 120 tappedsecondarywinding 330', the 120- spaced cmp'onents of secondary 330 beingconnectedin delta. If desired, winding 33a may be made axially ad-'justable in accordance with the transformer structure disclosed in Fig.8'. However, in lieu of the wave generator 32 utilized in the arrange-'-ment shown in Fig. 1 excitation for the primary winding 33a' is obtainedby tapping on to the 975 cycle output terminals of oscillator I l.variable resistor 52' can be used inthe circuit between the primary 33aand the 975 cycle sourceto obtain any adjustment in current that maybenecessary. The three output conductors leading from the taps on thesecondary winding 33c" feed into the electricalconnections betweenflux-gate iii and the stator windings l'iiof coupling Autosyn it in thesame manner'as shown in thearrangement in Fig. 1. These conductors haveaccordingly been identified in Fig. 4' by reference" numerals 35', 3tand 31". Resistor units similar to resistors 38436, inclusive, in Fig; 1can also be included in circuit with the conductors 3'5""41", inclusive,but these have not been shown.

Also included in Fig. 4 is a wave generator 43 which is essentiallysimilar to any of the three three primaries of fiux-gate HT and caninclude" a variable resistor M. for making any changes in. primarycurrent which may become necessary. A variable resistor can also be usedin the secondary 43d if desired.

Wave generator 43, in one form of the invention, can be maintained atall times in a vertical position with reference to the carrier, such asa ship. There is no permanent-magnet device present in wave generator 43and hence the voltage induced in the secondary Md is caused solely bythe earths magnetic field as modified by the car rier; As the carriermoves in latitude, the vertical component of the earths' field variesand 1 thus a varying output potential from secondary 43d is obtained.This output is then connected in parallel with the primary winding- 33d'of transformer 33' along with the other, and fixed, 975'cycle source.The arrangement shown in Fig. 4 works in substantially the same manneras: that shown in Fig. 1 which has already been described in detail.However, in addition to providing the necessary correction for deviationat any one latitude, which is the result obtained by the arrangement'shown in Fig. l, the arrangement in Fig. 4, in addition, will alsocompensate automatically for changes in the deviation characteristic ofthe ship caused by changes in latitude. It will be noted that thevertical generator 43 supplies in this instance the same type ofcorrection provided by the Flinders bar used in the-magneticcompass art.

Alternatively, device 43 may be fixed vertically with reference to theearth as by stabilizing it with a gyro. If this arrangement is used,device 43 may be fixed to the same gyro as flux-gate iii in a mannershown diagrammatically in Fig. 3. In this position, device 43 actssomewhat in the manner of a Flinders bar to correct for changes in thevertical component of the earths magnetic field with changes in latitudeand can also compensate part of the heeling error.

Asa further alternative arrangement, the three component parts of thedistributed secondary winding 330' of transformer 33' may be\'(-connected instead of connected in delta as shown in Fig. 4. Also,the fixed 975- cycle source maybe" the wave generator 32 of Fig. 1.

In an actualv correction problem the curve of compass deviation such asshown in Fig. 5 may be obtained by any of the standard well-known and(b) the points of maximum amplitude. the

ship is swung so as to be on the heading of anull, and the rotatableprimary 33a or 3321 of the correcting. transformer 33 or 33 is.positioned so.

that. there is no deviation inthe compass systemfor. thisv particularheading. The ship is then swungv to the heading of a point ofmaximumdeviation and the primary excitation of the-cor rectortransformer isadjusted' toremove all deviation; it will then be foundthat the deviation will have been removed or reduced to a small valuefor all headings of the ship. Should finer adjustment be desired asecond: swing. or the ship: can be madeand the controls readjusted.Should the curve of compass variations be non-syme' metrical, it may benecessary to adjust the re--is.excited-.fronrthe:sameflficyclasourceqas'the resistances in thesecondaryleads: from: the cor rector transformer, and/ or impedance inthe primary excitation circuit.

In a system including vertical wave generator 43, the relative amountsof voltage supplied by that generator and by the fixed source will beadjusted in such a manner that the proper change in total effect will beachieved for a certain change in magnetic latitude. This can beaccomplished by making actual settings in the two different latitudes,or by artificial means approximating such changes.

Many ships encounter the necessity for compensating the compass undertwo widely different conditions. Examples are found in the use ofdegaussing coils, and in the loaded and unloaded state of a shipcarrying a magnetic cargo, e. g., tanks. For such installations, Icontemplate providing two separate compensating systems selected by asuitable switching arrangement.

In some applications of this invention, it may be desirable to obtaingreater flexibility in generating curves than that offered by the singletransformers 33 or 33 shown in Figs. 1 and 4, respectively. For example,it may be desirable to compound two curves of varying amplitude andfrequency. This increased flexibility can be obtained by the arrangementshown diagrammatically in Fig. 8 wherein the correction transformercomprises a primary winding 13a and a pair of secondary windings 13b,130. Primary winding 13a corresponds with primary 33a of transformer 33and is so supplied that it can be rotatably adjusted in either directionand also vertically adjusted either up or down. The secondary windings13b and 13a are similar in construction to the secondary winding 33c oftransformer 33 except that one of them, 13b, is supported for rotationin either direction and also vertically adjustable, the other winding130 being fixed in a given position. The three output leads at the 120tapped points on the secondary windings 13b and 130 are connected inparallel and, for the compass application of the invention as shown inFig. 1, would feed over conductors 35, 36 and 31 to the stator winding lof coupling Autosyn 16.

It is evident that the arrangement shown in Fig. 8 could be appliedequally as well to any other self-synchronous transmission system and insuch case conductors 35, 36 and 31 would be tapped into the connectionsbetween the armature windings of the transmitter and repeater units ofthe system. The secondary windings 73b and 130 may be either connected Yor delta.

Another means of controlling the shape of the correction curve would beto utilize phase shifting devices in the primary circuit of thecorrection transformers such as the variable capaciter 74 shown in Fig.8.

The correction transformer herein described is claimed in my copendingapplication, Serial No. 69,546, filed January 6, 1949, and entitledMotion Transmission System.

Although the foregoing illustrations of the invention representpreferred embodiments thereof, various changes can be made therein bythose skilled in the art without departing from the spirit and scope ofthe invention as defined in the appended claims.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

Having thus fully described my invention, I claim:

1. A system for transmission of angular motion comprising a transmittingdevice, a repeating device having a field element and connected toreceive voltages from said transmitting device, and means for modifyingthe voltages impressed upon the field element of said repeating devicefrom said transmitting device in accordance with the instant angularposition of said transmitting device, said voltage modifying meanscomprising a transformer, the primary and secondary of which are movablerelative to each other, and adjustable impedance means combining thesecondary voltage output with the voltage output from said transmittingdevice.

2'. The system as in claim 1 wherein said voltage-modifying means isconnected in shunt across said conductive connections.

3. In a telemetric system a transmitter and a receiver, said transmitterhaving an element movable relative to an external reference, saidreceiver having a movable member and a stationary winding, conductorsinterconnecting said transmitter and said receiver whereby relativemovement of said transmitter element is effective to cause movement ofthe movable member of said receiver, and means interposed in saidconductors for effecting asynchronous operation between said transmitterand said receiver, said last-named means comprising a rotary transformerhaving a stator, and variable impedance means connecting said statoracross said con ductors.

4. In a telemetric system a transmitter and a receiver, said transmitterhaving an element movable relative to an external reference, saidreceiver having a rotatable member and a stationary winding, conductorsinterconnecting said transmitter and said receiver, whereby relativemovement of said transmitter element is effective normally to causemovement of the rotatable member of said receiver in sychronism withsaid transmitter element, a transformer, and variable resistance meanscoupling said transformer to said conductors for effecting asynchronousoperation between said transmitter element and said receiver member. a

I 5. An earth-inductor-compass system comprising, earth-induction meansfor deriving from the horizontal component of the earths magnetic fielda first electromotive force which is variable in accordance with theamount and direction that said induction means are displaced in azimuthrelative to the direction of the earths magnetic field, means forproducing a correction electromotive force which is variable as afunction of the angular displacement of said induction means in order tocorrect for the distortion of the earths magnetic field caused by themagnetic qualities of the craft upon which the system is mounted,electrically operated indicator means for indicating the direction ofthe earths field, and means for applying both said first-and correctionelectromotive forces to said indicator means whereby the latter willindicate substantially the true direction of the earths field.

6. An earth-inductor-compass system comprising earth-induction means forderiving from the horizontal component of the earths magnetic field anelectromotive force varying in accordance with the amount and directionthat said induction means are displaced in azimuth relative to thedirection of the earths magnetic field,

electrically operated indicator means for indicating the angulardisplacement of said induction means, electrical connections betweensaid induction and indicator means, and means for providing a secondelectromotive force for modifying the electromotive force produced bysaid induction means as a function of the angular displacement of saidinduction means.

7. An earth-inductor-compass system compris ing earth-induction meansfor deriving from the horizontal component of the earths magnetic fieldan electromotive force varying in accordance with the amount anddirection that said induction means are displaced in azimuth relative tothe direction of the earths magnetic field, electrically operatedindicator means for indicating the angular displacement of saidinduction means, electrical connections between said induction andindicator means, and means for modifying the electromotive forceproduced by said induction means, last said means including inductivemeans responsive to the vertical component of the ambient magnetic fieldfor producing therefrom an electromotive force automatically variablewith variation in magnitude of the vertical component of the earthsmagnetic field.

8. An earth inductor-compass system compris ing a polycircuit earthinduction means having primary and. secondary windings for deriving fromthe horizontal component of the earths magnetic field electromotiveforces varying in accordance with the amount and direction that saidinduction means are displaced in azimuth relative to the direction ofthe earths magnetic field, polycircuit electrical indicator means forindicating the angular displacement of said induction means, electricalconnections between said induction and indicator means, and means formodifying said electromotive forces, last said means comprising atransformer having a single circuitprimary and a polycircuit secondary,means connecting the output from said polycircuit secondary of saidtransformer into said electrical connections, wave generating meansresponsive to an adjustable relatively strong undirectional magneticfield for producing an electromotive force alternating in synchronismwith the Varying electromotive forces derived from the secondary of saidinduction means, and means coupling said alternating electromotive forceto said transformer primary.

9. A remote-reading compass system comprising, a compass means forproducing an electromotive force varying in accordance with the amountand direction that the compass of said system is displaced in azimuthrelative to the direction of the earths magnetic field, indicator meansoperated by said varying electromotive force for indicating the angulardisplacement of said compass, and means for providing a secondelectromotive force for modifying said electromotive force as a functionof the angular displacement of said compass.

10. An earth-inductor-compass system comprising, a fluxgate, electricalindicator means connected to the output of the secondary of saidfluxgate for indicating the azimuthal position of said fiuxgate, asource of alternating current for the primary of said fluxgate, acontrol transformer, means combining the output of the secondary of saidcontrol transformer with'the out put from the secondary of said fluxate, d means for exciting the primary of said control transformer withalternating current synchroe iii) i2 nized with the alternatingcurrent'exciting the primary of said fluxgate and at a frequency equalto the output frequency of said fiuxgate.

11. An earth-inductor-compass system comprising, a fiuxgate having aplurality of interconnected fiuxgate legs, each leg having primary andsecondary windings thereon, electrical indicator means connected to theoutput of the fluxgate secondary windings for indicating azi muthalposition of said fluxgate, a-control trans former having primary andsecondary windings, a Wave generator comprising a single fluxgate leghaving a primary and secondary winding and operating in a relativelystrong unidirectional magnetic field, means connecting the output fromthe secondary winding of said wave generator to the primary winding ofsaid control transformer, means for supplying alternating current to theprimary windings of said fluxgate and wave generator from a commonsource, and means connecting the output from the secondary winding ofsaid control transformer in parallel with the output from the secondarywindings of said fiuxgate.

12. An earth-inductor-compass system com.- prising, a fluxgate having aplurality of interconnected fiuxgate legs, each leg having primary andsecondary windings thereon, electrical in dicator means connected to theoutput of the fluxgate secondary windings for indicating azi muthalposition of said. fiuxgate, a control. transformer, an oscillator havinga pair of outputs, one of said outputs being at a frequency double thatof the other output, means connecting the lower frequency output to theprimary windings of said fluxgate, means connecting the higher frequencyoutput to the primary winding of said control transformer, and meansconnecting the secondary winding of said control transformer in parallelwith the output of the iiuxgate secondary windings.

13. An earth-inductor compass system comprising, a fluxgate, electricalindicator means connected to the output of the secondary of saidfluxgate for indicating the azimuthal posi tion of said fluxgate, asource of alternating current for exciting the primary of said fluxgate,a control transformer, means combining the output of the secondary ofsaid control transformer with the output from the secondary of saidfluxgate, means for providing the primary of said control transformerwith a first alternating current at a frequency double that of thealternating current exciting the primary of said fluxgate, and means foralso providing the primary of said control transformer with a secondalternating current at a frequency double that of the alternatingcurrent exciting the primary of said fluxgate, said second alternatingcurrent being variable with variation in magnitude of the verticalcomponent of the earths magnetic field.

14. An earthi-inductor compass system comprising, earth induction meansincluding a plurality of saturable core type transformers for producingfrom the horizontal component of the earths magnetic field electromotiveforces: varying in accordance with the amount and di-- rection that saidinduction means are displaced. in azimuth relative to the direction ofthe earths magnetic field, a source of alternating current for excitingthe primaries of said transformers, means interconnecting thesecondaries of said transformers to provide a polycircuit electromo-'tive force output, electrical indicator means including a polycircuitwinding for. indicating,

angular displacement of said induction means, electrical connectionsbetween the secondaries of said transformers and the polycircuit windingof said indicator means, and means for modifying the polycircuit outputproduced by said induction means as a function of the angulardisplacement of said induction means to compensate for deviation, lastsaid means comprising a control transformer having a single circuitprimary and a polycircuit secondary, means connecting the polycircuitoutput from the secondary of said control transformer into theconnections between said induction and indicator means, and means forexciting the primary of said control transformer with an alternatingcurrent at a frequency double that of the current feeding the primary ofsaid earth induction means.

15. An earth-inductor compass for use on a mobile craft comprising, aninductor element for generating a first alternating-current signalrepresentative of the resultant of the earths field and any extraneouspermanent field, means for deriving at least one auxiliary signalsynchronous with said first signal and equal in amplitude and oppositein phase to that of a component of said first signal due to saidextraneous permanent field, and means for modifying said first signal bysaid auxiliary signal to develop a signal the phase of which isrepresentative of the true bearing of the craft.

16. A remote-reading compass system comprising, means for producing afirst electromotive force varying in accordance with the amount anddirection that the compass of said system is displaced in azimuthrelative to the direction of the earths magnetic field, means forproducing a correction electromotive force in order to correct for thedistortion of the earths magnetic field caused by the magnetic qualitiesof the craft upon which the system is mounted, electrically operatedindicator means for indicating the direction of the earths field, andmeans for applying both said first and correction electromotive forcesto said indicator means whereby the latter will indicate substantiallythe true direction of the earths field.

17. An earth-inductor compass for use on a mobile craft, comprising, aninductor element for generating a first alternating-current signalrepresentative of the resultant of the earths field and any extraneouspermanent field, means for deriving at least one auxiliary signalsynchronous with said first signal and equal in amplitude and oppositein phase to that of a component of said first signal due to saidextraneous permanent field, and means for modifying said first signal bysaid auxiliary signal to develop a signal having an electricalcharacteristic which is representative of the true bearing of the craft.

18. An earth-inductor-compass system, comprising a multi-leg fiuxgate,indicator means connected to said fiuxgate for indicating the azimuthalposition of said fiuxgate, a source of alternating current forenergizing said fiuxgate, means including a control transformer having aplurality of generally angularly related secondary windings and arelatively movable primary winding and adapted to be energized withalternating current from said source, and means combining the output ofthe secondary windings of said control transformer with the output ofangularly corresponding legs of said fluxgate.

19. The system as defined in claim 18 wherein said control transformerdevelops a correction voltage that is variable in accordance with vari-14 ation of position of said fiuxgate, and said combining meanscomprises a plurality of variable resistors.

20. The system as defined in claim 18 wherein one of said windings isadapted to be maintained in a predetermined fixed position relative tothe other winding.

21. A system for transmission of angular motion comprising atransmitting device, a repeating device having a field element andconnected to receive voltages from said transmitting device, and meansfor modifying the voltages impressed upon the field element of saidrepeating device from said transmitting device in accordance with theinstant angular position of said transmitting device, saidvoltage-modifying means comprising a transformer having a primary and atleast two secondaries movable relative to the primary, and meansconnecting the secondaries into the connections between saidtransmitting and repeating devices.

22. In a data-transmission system, a field-direction signalling devicehaving means to generate a signal according to the instant position ofsaid device relative to an external field, a repeater unit having fieldand armature elements, one of said elements constituting the rotor andsaid other element constituting the stator, means electricallyconnecting said signal-generating means to said armature element,whereby said rotor is normally positioned in accordance with therelative instant position of said device, means including a transformerhaving primary and secondary windings, one of said windings beingcoupled to said connecting means through adjustable impedance elementsfor applying a correction signal thereto, thereby to modify the signalimpressed upon the armature element from said device as a function ofsaid instant relative position.

23. The combination as in claim 22, further comprising means forindependently adjusting said impedance elements thereby to alter theamplitude of said correction signal.

24. The combination as in claim 22, wherein the other of saidtransformer windings is connected to a source of alternating potentialwith adjustable impedance means in the circuit to said source forcontrollably adjusting the magnitude of the voltage impressed on saidother winding.

THOMAS OCONN ELL MCCARTHY.

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